Soil gas prospecting for petroleum



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atent SEARCH ttuum assarss Patented Feb. 7, 1967 fire 3,302,706 SOIL GASlROSPECTlNG FQR PETROLEUM Robert R- Thompson, Tulsa, Gltlm, assignor toPan American Petroleum (Iorporation, Tulsa, 01th., a corporation ofDelaware 1 N Drawing. Filed Feb. 21. 1964, Ser. No. 346,416

Claims; (Cl. 166-4) This invention relates to recovering oil fromsubsurface formations. More particularly, it relates to determiningwhich of several possible formations underlying a certain area containsoil so a well can be drilled to that forma tion to recover the oil.

In the prior art many methods have been proposed for determining whetheran oil producing formation underlies a particular area. Several of thesemethods involve obtaining soil gas samples from the soil over the areaof interest and analyzing these samples for hydrocarbon content. Ofter aplot of the results ofv such a survey indicates that an oil-bearingformation lies under at least a portion of the area. The question thenis one of how deep below the earths surface is the oil-bearingformation. An answer to this question is desirable in order to decidehow deep to drill in order to recover the oil. In many areas it will beknown that several formations underlying the area not potentiallyoil-bearing formations. The question is usually which of these is theone which is actually oilbearing.

An object of this invention is to determine which of several potentiallyoil-bearing formations below an area contains the oil indicated by asoil gas analysis survey so that a well can be drilled to thisparticular formation to aid in recovering the oil. Still other objectswill appear from the following description and claims. 7

I have now found that the soil gases from each formationhave acharacteristic ratio of various hydrocarbons- This is particularly truefor a region of limited extent where the depth of each formation israther constant. The two hydrocarbons with which I prefer to work aremethane and ethane since these are generally present in the largestamounts. In order to avoid possible mislead ing results from methanegenerated near the surface, however, it is sometimes advisable to useratios of higher ;molecular weight hydrocarbons, such as propane and thebutanes. Surface effects can usually be avoided by taking samples fromgreater depth. In addition, two potential oil-bearing formations belowthe area of interest may have the same ratioof methane to ethane butdifferent ratios of, say, isobutane to n-butane. In such cases it willbe desirable to use the hydrocarbon ratio which is different for thevarious potential oil-bearing formations.

The technique is simply to determine the ratio of two particularhydrocarbons in soil gases obtained over fields .known to be producingfrom the'various potential oilbearing formationsunderlying the surveyarea. The ratio of the same two hydrocarbons is then measured in soilgases obtained over the survey area. A well is then drilled to aformation producing the soil gas hydrocarbon ratio like that over thesurvey area.

Soil gas samples can be taken directly from the soil in place. Forexample, a tube may be introduced to the desired depth and a sample ofthe gases present can be withdrawn. It is greatly preferred, however, totake soil samples and release gases from these soil samples by treatmentwith acid. After carbon dioxide is removed by an alkaline wash, the gassample is analyzed by a method of gas chromatography. The same analysistechnique should preferably be used with soil gas whatever the means bywhich the soil gas is derived. In the preferred gas chromatographytechnique, a stream of inert gas, such as nitrogen, continuously flowsthrough a tube filled with an adsorbent material such as silica oralumina. The soil gas is introduced as a batch or slug which the inertgas sweeps through the column. The rate of travel of the varioushydrocarbons varies with molecular weight so they leave the tube atdifferent characteristic times. The quantity of each can then bedetermined by ionization in a hydrogen flame, by combustion followed bymeasurement of combustion products, or by other techniques, and theratios of the various hydrocarbons can be directly determined. Stillother techniques for measuring hydrocarbons in soil gases have beendescribed in the past and can be used but the chromatographic techniqueappears to be much more accurate and is greatly preferred.

An important correlation has been noted during surveys by the proposedtechnique. This is a correlation between the density of oil in anoil-bearing formation and the ratio of hydrocarbons in soil gases abovethe formation. There is a substantially straight-line relationshipbetween the methane-to-cthane ratio and the API gravity of the oil inthe subsurface oil-bearing formation. This relationship is representedby the formula G=65-3.7R where G is the API gravity of the oil and R isthe methane-toethane ratio. The API gravity of the oil from producingfields is known. Therefore, this relationship will usually be sufiicientto indicate the oil-producing formation under a particular portion of asurvey area. This eliminates the necessity for obtaining soil gassamples over known fields and determining methane-to-ethane ratios ofsuch samples. The oil densities from at least two potential producingformations must, of course, be different for this technique to beapplicable.

One precaution which should be noted is that interpretation of the datashould take into account unusual circumstances such as changes in thedensities of oils in any formation in question, either in the surveyarea or the field used as a standard. in general, the formation in thestandard field and in the survey area should be at as nearly the samedepth and as near together geographically as possible.

Another variable which should be considered in making interpretations isthe nature of the soil from which samples are taken. An advantage of mymethod, however, is that the nature of the soil seems to make much lessdifference in hydrocarbon ratios than it does in total hydrocarboncontent.

Still-another factor considered important by some woi'lo ers is the timeat which soil gas samples are taken over known fields. Some workers havenoted differences, at least in total hydrocarbon content, betweensamples taken before the field is produced and after the field hasproduced for a few months or years. To avoid as much as possiblediificulties from this factor, it is preferred that soil gas samples betaken over known fields as soon as. possible after production from thefield has started.

The results of a survey over a wide area in Wyoming will illustrate useof my process. The survey covered an area below which three potentialproducing formations were known to exist. These were the Muddy, theDakota, and the Minnelusa formations. The hydrocarbon content of thesoil gas was high over several areas. Methane-toethane ratios weredetermined and plotted on a map with the'total hydrocarbon content ofthe gas.

Over one of the larger high areas, the methane-to-ethane ratio was alsohigh. This ratio averaged about 12, but ranged from about 9 to about 17.The same was true of an area adjacent an old field. The old fieldproduced from the Minnelusa formation. The API gravity of the producedoil was about 22. When the large area was drilled, a Minnelusa field wasfound which also produced oil having a gravity of about 22. The areaadjacent the old field was also drilled and also produced from theadjacent to an existing fieldthis time a small one. The

soil gas survey indicated that this field. could be extendedconsiderably with new production from the Dakota or Muddy according tothe hydrocarbon ratios.

The existing field in this case produced from the Dakota formation. Theproductive area was later extended-as predicted with new wells producingfrom the Dakota. f the other two areas, one produces a 39 gravity oilfrom the Muddy formation. The other produces an oil of about 38 APIgravity from the Dakota formation.

It will be noted that the gravities of the two crude oils from the Muddyand Dakota formations were almost the same. The methane-to-ethane ratioswere also so nearly the same that the sources of the soil gas could notbe distinguished from this ratio alone. Average values for samples overone area and 13 samples over the other gave methane-t0-ethane ratioswhich were very nearly identical. The variations in the analyses ofindividual samples, as noted above, show the importance of usingaverages for large numbers of samples in determining the oil gravity andtherefore the formation serving as a. source for the soil gases. Thesame averages showed a difference of about '6 percent in the ratio ofbutane to isobutane. Whether a difference of this magnitude is reallysignificant in this case is not known. The difference does illustrate,however, how some ratios may be identical over two areas while otherratios may be different. In such cases, the ratio which differs overthetwo known fields should be used in determining the source of soil gasesin the survey area. There will, of course, be some cases in which nodetectable differences will be apparent between soil gases over twofields producing from different formations. In this case my process canbe used to exclude most of the possible producing formations and narrowthe possibilities to these two formations.

Plotting not only total hydrocarbon content, but also ratios ofhydrocarbons on'a map of the survey area is usually advisable. The plotof total hydrocarbon contents of soil gas samples will generally besufficient alone to indicate by a high spot or a halo, for example,where a well should be drilled. A plot of hydrocarbon ratios not onlydistinguishes between productive: subsurface zones closely spacedhorizontally, although in different formations, but also aids toindicate the most favorable locations for drilling wells into suchzones. For this purpose, the maps of hydrocarbon ratios may even. be

contoured in cases where there is some variation in hydrocarbon.ratioswithin small portions of the survey area.

The hydrocarbon ratios can be put to several important uses. Forexample, they can be used to determine whet-hero. surface soil gasanomaly indicates a possible field large enough to be of economic value,or if the surface anomaly actually can be broken down into two or moresmaller merging or overlapping anomalies caused by two or moresubsurface accumulations too small to be of significant economic value.

In addition, the ratios can be used to aid in interpreting soil gasresults in. areas where complex and overlapping or merging surfaceanomalies are found because petroleum reservoirs in different subsurfaceformations occur closely spaced laterally.

The principal use, as noted previously, is to determine which subsurfaceformation will be productive within a given surface soil gas anomaly soa well can be drilled to that formation to recover the oil.

While recovery of petroleum from a well drilled in the survey area isthe most probable use to which the well will be put, it is possible, ofcourse, that the well may be drilled and used as an injection well in agas drive, water drive, or the like to recover the oil.

Many other possibilities will occur to those skilled in the art. I donot, therefore, wish to be limited to the examples given above, butonlyby the following claims.

I claim: a

1. A method for recovering oil from a subsurface geological formationcomprising analyzing soil gas samples taken over a survey area todetermine the total hydrocarbon content and the ratio of two differenthydrocarbons in said samples, plotting the total hydrocarbon content andthe ratio of said two different hydrocarbons on a map of said surveyarea, determining the ratio of said two different hydrocarbons in soilgas samples taken over fields producing from formations from whichproduction might be expected below said survey area, and drilling "awell within said survey area at a location indicated to be promising bythe total hydrocarbon contents of samples taken over said survey area,said well being drilled to a formation producing a ratio of said twodifferent bydrocarbons like the ratio determined over that portion ofthe survey area in which said well is drilled- 2. The method of claim 1in which said two different hydrocarbons are methane and ethane.

3. The method of claim 1 in which the soil gas samples are analyzed bygas chromatography.

4. A method for recovering oil from a subsurface formation in a surveyarea in which there are at least two potential producing formations eachknown to produce, in nearby areas, oil having an API gravity differingfrom the API gravity of oil produced from at least one other of saidpotential producing formations, comprising analyzing soil gas samplestaken over said survey area to determine the total hydrocarbon contentand the methaneto-ethane ratio in said samples, plotting the totalhydrocarbon content and the methane-to-ethane ratio on a map of saidsurvey area and drilling a well within said survey area at a locationindicated to be promising by the total hydrocarbon contents of samplestaken over said survey area, said well being drilled to a formationproducing oil having an API gravity indicated approximately by theformula G=65-3.7R in which G is the API gravity of the oil and R is themethane-to-ethane ratio determined over that portion of the survey areain which said well is drilled.

5. The method of claim 4 in which said soil gas samples are analyzed bygas chromatography.

6. A method for recovering oil from a subsurface geological -formationcomprising analyzing soil gas samples taken over a survey area todetermine the total hydro carbon content and the ratio of two differenthydrw carbons in said samples, plotting the total hydrocarbon contentand the ratio of said two different hydrocarbons on a map of said surveyarea, determining the ratio of said two different hydrocarbons in soilgas samples taken over fields producing from formations from whichproduction might be expected below said survey area, and drilling twowells within said survey area at locations indicated to be promising bythe total hydrocarbon contents of samples taken over said survey area,one well being drilled to a first formation producing a ratio of saidtwo different hydrocarbons like the ratio determined over that portionof the survey area in which said one well is drilled, and the other wellbeing drilled to a second formation producing a ratio of said twodifferent hydrocarbons like the ratio determined over that portion ofthe survey area in which said other well is drilled.

7. The method of claim 6 in which said two different hydrocarbons aremethane and ethane.

8. The method of claim 6 in which the soil gas samples are analyzed bygas chromatography.

9. A method for recovering oil from a subsurface formation. in a surveyarea in which there are at least two potential producing formations eachknown to produce, in nearby areas, oil having an API gravity differingfrom the API gravity of oil produced from at least one other of saidpotential producing formations, comprising analyzing soil gas samplestaken over said survey area to determine the total hydrocarbon contentand the methaneto-ethane ratio in said samples, plotting the totalhydrocarbon content and the methane-to-ethane ratio on a map of saidsurvey area, and drilling two wells within said survey area at locationsindicated to be promising by the total hydrocarbon contents of samplestaken over said survey area, the wells being drilled to differentformations producing oils having API gravities indicated approximatelyby the formula G=653.7R in which G is the API gravity of an oil and R isthe methane-toethane ratio determined over that portion of the surveyarea in which a well is drilled. v

10. The method of claim 9 in which said soil gas samples are analyzed bygas chromatography.

References Cited by the Examiner UNITED STATES PATENTS 2/1943 Smith23-230 X 2,312,271 3,196,664 7/ 1965 Teal 73-23.l

FOREIGN PATENTS 578,811 6/1959 Canada.

OTHER REFERENCES Scott, R. P. W. Gas Chromatography, Butterworths;

. 1960, pp. 3.31 and 332.

ERNEST R. PURSER; Primary Examiner.

1. A METHOD FOR RECOVERING OIL FROM A SUBSURFACE GEOLOGICAL FORMATIONCOMPRISING ANALYZING SOIL GAS SAMPLES TAKEN OVER A SURVEY AREA TODETERMINE THE TOTAL HYDROCARBON CONTENT AND THE RATIO OF TWO DIFFERENTHYDROCARBONS IN SAID SAMPLES, PLOTTING THE TOTAL HYDROCARBON CONTENT ANDTHE RATIO OF SAID TWO DIFFERENT HYDROCARBONS ON A MAP OF SAID SURVEYAREA, DETERMINING THE RATIO OF SAID TWO DIFFERENT HYDROCARBONS IN SOILGAS SAMPLES TAKEN OVER FIELDS PRODUCING FROM FORMATIONS FROM WHICHPRODUCTION MIGHT BE EXPECTED BELOW SAID SURVEY AREA, AND DRILLING A WELLWITHIN SAID SURVEY AREA AT A LOCATION INDICATED TO BE PROMISING BY THETOTAL HYDROCARBON CONTENTS OF SAMPLES TAKEN OVER SAID SURVEY AREA, SAIDWELL BEING DRILLED TO A FORMATION PRODUCING A RATIO OF SAID TWODIFFERENT HYDROCARBONS LIKE THE RATIO DETERMINED OVER THAT PORTION OFTHE SURVEY AREA IN WHICH SAID WELL IS DRILLED.